This invention relates generally to signal processing methods and circuits for overcoming noise inherent in a transmission or recording medium and more particularly concerns compandors comprising composite compressors and composite expanders for achieving that result.
Satellite linked communication networks have come into wide use within the past several years for cable television, security systems, private video and audio communications, and for numerous other purposes. In satellite linked cable television distribution, for example, additional subcarriers are often provided within various video channels in order to carry high fidelity, stereophonic audio signals to cable subscribers. Multiplexing techniques are used with the additional subcarriers to increase the amount of information carried by each subcarrier. In such satellite linked communication networks, the noise inherent in the radio transmission link or medium between the ground station and the satellite must be overcome if high quality service is to be provided to the subscribers.
Compressors and expanders (compandors) have been used with satellite radio links to overcome the effects of noise in the satellite radio link. Compandors can also be used to reduce the subcarrier bandwidth required for FM transmission by allowing a reduction in the frequency deviation of the transmitted signal.
Compressors at the transmitter end of the radio link over-amplify the low amplitudes of a normal analog signal and under-amplify the high amplitudes of the normal analog signal. In the resulting compressed analog signal, the over-amplified low amplitude signals thus can overcome the noise on the transmission medium while the under-amplified high amplitude signals do not overload the transmission medium. Expanders at the receiver end of the radio link reverse the compression process by over-amplifying the high amplitude signals and under-amplifying the low amplitude signals to reconstruct the normal analog signal.
The prior art discloses various noise supression and reduction systems using compandors. U.S. Pat. No. 2,173,472 to Doba discloses dividing the normal analog signal into two or more frequency bands or frequency components, compressing each band separately, transmitting each band over a separate transmission medium, expanding each band separately and then recombining the two bands to reconstitute the original analog signal. The use of two transmission media, one for each frequency band, is impractical for satellite radio links. Moreover, a gain error in either transmission medium will produce a frequency response error at the receiver end expander.
In order to use a single transmission medium, U.S. Pat. No. 2,358,045 to Barney discloses a system like Doba's except the Barney system uses tandem or series connected compressors and expanders, one for the high frequency band followed by one for the low frequency band. In the Barney system, a gain error in the transmission medium will still, like Doba, produce a frequency response error at the receiver end.
In a compandor system such as Doba or Barney, a frequency response error results from a gain error in the transmission medium because the compressor provides a family of frequency response curves having a different response for each amplitude of input signal. The expander also provides a complementary family of frequency response curves having a different response for each amplitude of input signal. If a gain error occurs during transmission in such a system, the input signal at the expander is shifted in amplitude, and the expander operates on a frequency response curve that is not the complement of the compressor's frequency response curve that produced the compressed signal in the first place. Therefore, a frequency response error results from the gain error in the transmission medium where only one frequency component of the normal signal is compressed for transmission (Doba) or where both frequency components of the normal signal are compressed in series (Barney) so that the frequency response errors do not cancel out.
Other prior art compandor systems, including that sold by the assignee of the present invention, compress only the high band signal and transmit the low band signal without being compressed. Such systems, like Doba and Barney, will experience frequency response errors where there is a gain error in the transmission medium. As a result, in such systems the compression ratio has been limited to avoid frequency response errors with the accompanying sacrifice in noise suppression due to the lower compression ratio.
At least one prior art compandor system compresses the whole frequency spectrum with a single compressor in an effort to eliminate frequency response errors resulting from gain errors. Such an approach, however, may experience unwanted audible noise modulation effects.